Class Fleet

Fleet object is responsible for moving a group of ship around the map in an ordered manner, that is:
1. provide a single move callback for a fleet as a whole,
2. resolve self-blocks in a group of ships
3. resolve MoveNotPossible exceptions.

move(self,
destination,
callback=None,
ratio=1.0)

ratio - what percentage of ships has to reach destination in order for
the move to be considered done: 0.0 - None (not really useful,
executes the callback right away) 0.0001 - effectively ANY ship
1.0 - ALL of the ships 0.5 - at least half of the ships etc.